The present invention relates to a polymeric actuator, comprising                a first and a second electrode layer, both containing electrically conductive material and able to change size in at least one direction of deformation, by charge injection or ion intercalation, and        a solid polymer electrolyte layer interposed between said first and second electrode layer, in which said solid polymer electrolyte layer is electrically insulating and ionically conductive,        where said actuator is able to deform by the action of the dimensional changes of said first and second electrode layer.        
Actuators able to generate force and displacement in response to an electrical signal are currently in use in various sectors of industry and the performance requirements are tending to be more and more diverse. A great many technologically advanced applications require actuators that are light, compact and actuated at low voltages. Moreover, it is important for said actuators to be able to effect movements easily in different directions, and moreover to be safe and easy to manipulate.
There has recently been increasing interest in organic actuators that can be deformed by an electrical signal. Many of said actuators are light and are able to operate in a gaseous environment, for example in the atmosphere. An example of said actuators is described in U.S. Pat. No. 7,315,106, which relates to an actuator of the type defined at the beginning. This actuator is composed of carbon nanotubes, a non-volatile ionic liquid and a polymer. Although this type of actuator is simple to manipulate and can be used in a gaseous environment, its laminar shape limits its directions of bending, as is described in U.S. Pat. No. 7,449,818.
Polymeric actuators with a tubular geometry have recently been reported. “High performance conducting polymer actuators utilizing a tubular geometry and helical wire interconnects” [1] describes a tubular electromechanical actuator based on polypyrrole with helical wire interconnects. Although the helical wire interconnects permit efficient charge injection/extraction without mechanical or electrochemical degradation of the system, this type of actuator does not allow a free choice of the directions of bending.
Polymeric actuators were recently proposed that can operate in air and in vacuum using an active layer composed of carbon nanotubes and an ionic liquid. The actuator comprises a conductive material (a gel composition of carbon nanotubes and ionic liquid), an electrode layer (comprising the conductive material and a polymer) and an ionically conductive layer (comprising an ionic liquid and a polymer). The conductive material displays good conductivity and good extensibility/contractility. The particular structure of said actuator makes it possible to use low operating voltages and it is stable both in air and in vacuum. Moreover, the production process is simple and makes it possible to obtain an actuator of extremely small dimensions suitable for a wide range of applications.
For example, a gel (called bucky gel [2]) prepared using single-walled carbon nanotubes (SWCNT) ground with ionic liquids (IL) based on imidazole was used for making a three-layered bendable actuator with an inner layer of ionic liquid electrolyte supported by polymer, interposed between electrode layers of bucky gel [3]. These actuators in gel have many favourable characteristics: they can operate in air for a long time without liquid electrolyte and require low voltages (3-4 V), the deformation is about 1% and the operating frequency greatly exceeds those obtainable with actuators based on carbon nanotubes that operate in a liquid environment. Moreover, they can be made easily by techniques of casting, rolling, moulding and spraying. The actuation of actuators based on nanotubes has been explained by effects of steric repulsion due to transfer of ions to the electrode [4] and by charge injection [5] which leads to a change in length of carbon-carbon bond of the nanotubes. As a result of charge injection, one of the electrodes expands while the other contracts or expands less than the other one, and in a three-layered configuration this produces the bending movement of the actuator.
The arrangement of a certain number of electrodes around a bar-shaped ionically conductive layer enables the actuator to perform complex bending and peristaltic movements.
U.S. 2002/195326 describes another actuator of the type defined at the beginning, comprising a first and a second electrode layer, both containing electrically conductive material, in particular polypyrrole, able to change size under the action of charge injection, and a layer of solid polymeric electrolyte, in particular a PMMA gel, interposed between the first and second electrode layer.
The existing devices designed for reproducing complex movements require a large number of units of polymeric actuators arranged in a predetermined configuration connected with particular mechanical and electronic requirements, to obtain bending and peristaltic movements in said actuator structure.